1. Field of the Invention
The present invention relates to a digital filter, a servo control unit, and a disk drive, and more particularly to a reduction in noise as filter characteristics are changed.
2. Description of Related Art
On the recording side of a magnetic disk X1 which is employed in a hard-disk drive (HDD) as a data storage medium, as shown in FIG. 2, concentric circular recording tracks X2 are formed. On the recording track X2, servo areas X3 are provided at predetermined angle intervals, and a servo pattern, an ID number, and the like are recorded on the servo areas X3 at a constant angular velocity. Between adjacent servo areas X3, there is provided a data area X4 on which data sectors X5 are recorded at a constant angular velocity. Also, in an HDD such as this, the data area X4 are segmented into some zones in the radial direction thereof, and the number of data sectors X5 corresponding to the radial position is recorded on each data zone, thereby making recording densities uniform.
In the aforementioned conventional HDD, if a sector is specified and recording/regeneration is instructed, the head X6 will be moved to a target track on which the specified sector for recording/regeneration has been recorded (seek control). After the head X6 has arrived at the target track, the head position is adjusted so that the head X6 follows the target track (following control).
In the servo control such as a seek control and a following control, a head arm X7 is moved based on the error between the current position of the head X6 and the position of a target track, by a hard disk controller (HDC) X9 provided in the HDD.
More specifically, the servo sector X3 records disk servo patterns (WEDGE-A, WEDGE-B, WEDGE-C, and WEDGE-D) and a cylinder ID number (CYLID), as shown in FIG. 3, and if the head X6 passes over the servo sector X3, the outputs of the servo patterns and ID number regenerated by the head X6 will be supplied to the HDC X9. If these regenerated outputs are supplied, the HDC X9 will compute the current position of the head X6 based on these outputs and will compute position error between the computed current position and the position of a target track. Based on the position error, the HDC X9 computes data (DACOUT value) for driving a voice coil motor (VCM) X8 which causes the head arm X7 to move.
In the seek control the HDC X9 computes the difference (position error) between the current position of the head X6 and the position of a target sector and then performs control based on this position error.
When a target track is specified, the HDC X9 first executes a control for accelerating the head X6 toward the target sector. Then, if the moving speed of the head X6 reaches a predetermined speed, the HDC X9 will execute a control for moving the head X6 at a constant speed. Finally, if the head X6 arrives near the target sector, the HDC X9 will execute a settling control for decelerating the head X6.
If the head X6 arrives at the target track, the HDC X9 will switch the settling control to a track following control for causing the head X6 to follow the target track. The control switching is performed by changing an arithmetic parameter which computes a DACOUT value.
The DACOUT value, obtained in the aforementioned way, contains the resonance frequency component of the actuator system comprising head arm X7. Therefore, if the VCM X8 is directly driven based on such a DACOUT value during the seek operation, there are cases where the head position control will become unstable. For this reason, for example, as shown in U.S. Pat. No. 5,325,247, the resonance frequency component of the DACOUT value is reduced to stabilize the servo control by employing a notch filter.
Also, if a low-pass characteristic is flat during the track following operation, there will be cases where control will not stabilize due to disk eccentricity and disturbance. For this reason, in background art, as shown for example in U.S. Pat. No. 5,608,586, the servo characteristic has a peak on the low frequency side so that control stability can be obtained.
In order to realize such control, the HDD is provided with a plurality of filters whose characteristics are different from one another. The filters are selected according to operating modes, or the operation of each filter is controlled.
However, if a plurality of filters are provided as previously described, the cost of the HDD will be increased.
For the above described reasons, a digital filter has been employed to change the filter characteristic by a parameter change. In the case where the aforementioned digital filter is employed however, when a parameter is changed, even if the filtering process attempts to continue by employing the immediately prior value of an interior variable as it is, an accurate value cannot be computed because the parameter has been changed, and the filter outputs will be discontinuous. Also, in the case where an interior variable is cleared when a parameter is changed, similarly an accurate value cannot be computed and therefore filter outputs will be discontinuous.
The present invention has been made in view of problems such as those described above, and accordingly, the object of the invention is to provide a digital filter, a servo control unit, and a disk drive which are capable of preventing filter outputs from becoming discontinuous when the filter characteristic is changed.
A digital filter according to the present invention comprises a first filter and initial value setting means. The first filter includes means for delaying an external filter input by at least a unit time, means for setting a filter coefficient by weighting at least either one or the other of the external filter input and the delaying means, and means for adding at least two of the external filter input, an output of the delaying means, and an output of the setting means to compute filter output. The initial value setting means sets the output of the delaying means, based on at least one of the filter input and the filter output before changing the filter coefficient, when the filter coefficient set by the setting means is changed.
The servo control unit according to the present invention comprises: servo arithmetic means for computing control output based on an external control input and band limiting means for setting a band limit to the control output computed by the servo arithmetic means. The band limiting means has the same structure as the digital filter of the present invention.